There is growing market demand for personal health and environmental monitors, for example, for gauging overall health and metabolism during exercise, athletic training, dieting, daily life activities, sickness, and physical therapy. However, traditional health monitors and environmental monitors may be bulky, rigid, and uncomfortable—generally not suitable for use during daily physical activity.
Sensors for detecting biometric signals, such as vital signs and other physiological information, are configured to isolate the biometric signals from other spurious signals and deliver biometric readings, such as heart rate, respiration rate, blood pressure, etc., to the user. Unfortunately, spurious signals that may be difficult to isolate from a biometric signal are associated with physical movement (e.g., physical exercise, such as walking, running, daily activities, etc.) of a sensor relative to the user or the environment of the user (e.g., sunlight, room light, humidity, ambient acoustical or electromagnetic noise, temperature extremes or changes in temperature, etc.).
For example, referring to FIG. 1, a conventional photoplethysmography (PPG) optical sensor 10 is illustrated that includes an optical source 14, and an optical detector 16. The optical sensor 10 is desirably positioned directly against the body B (i.e., the skin) of a subject wearing the optical sensor such that light L1 from the optical source 14 is directed into the body B and is subsequently detected by the optical detector 16. However, movement of the user can cause the sensor 10 to move relative to the body B such that light L2 can take a direct path from the optical source 14 into the optical detector 16 (for example by reflection off of the body, i.e., the skin, of the user), which increases spurious signals. Moreover, motion artifacts may cause the distance between the sensor 10 and body B to change in time, thereby modulating L1 and L2 in time, leading to motion artifact noise on the signal generated by the detector 16.
Previous ways of isolating heart rate signals from other signals include the use of passive and active signal processing algorithms, increasing optical sensor output and displacing the optical source from the photodetector, and pushing the sensor more firmly against the user so as to limit the effects of physical movement on the heart rate signal.